Field of Invention
The present invention relates to a laser induced nano-brazing method for reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates.
Description of Related Arts
Reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates, which has high specific strength, specific stiffness, dimensional stability, designability and excellent performance in resistance to abrasion, corrosion and radiation, is a new structural material which has the greatest application potential amongst the metal based composite materials and is widely used in different industries such as aerospace, automotive, instrumentation, electronic information and precision machinery. Reinforced aluminum matrix composite with high volume fraction (50%˜70%) of silicon carbide particulates, which has a coefficient of expansion close to the low coefficient of expansion of glass and a hardness close to the high hardness of diamond, is unique and useful in electronic packaging and key components in aviation and aerospace industries. For examples, in the advanced early warning aircraft, fighter jets, large phased radar, spaceships, space stations, Chinese lunar-orbiting spacecraft Chang'e and satellites, and military equipment such as ballistic missiles, which are currently in urgent need of development in China's national defense, the radar core components—T/R signal transmission and high power module receiver chip has a sharp increase in demand. Because Kovar (Fe—Ni—Co) and traditional packaging materials such as W/Cu and Mo/Cu, which are used for module package housing for a long time, have large specific weight, involve complicate manufacturing process and high cost, these materials are far from suitability for the highly lightweight design requirements of aviation and aerospace structure. As a result, there is an urgent need of a new electronic packaging materials which has the three characteristics of low expansion, high thermal conductivity and lightweight. Reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates is an ideal alternative for electronic packaging materials because not only its specific weight is ⅓ of Kovar while its expansion coefficient is close to ceramic substrate of packaged chip, its thermal conductivity is 10 times of that of Kovar.
Accordingly, there is a trend to use reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates to replace the traditional packaging materials such as Kovar, W/Cu and Mo/Cu in aviation, aerospace and electronics industries. Because
In the military equipment such as advanced early warning aircrafts, fighter jets, large phased radars, spacecraft, satellites, and ballistic missiles, since the number of electronic components being used is greatly increased, a decrease in weight will have an implication of an increase in flexibility and survivability in operation, a decrease in load of fuel and an increase in effective load, thereby resulting in significant saving for the country. As a rough estimate, an airborne warning and control system requires 40,000 unit of electronic package module, an interceptor fighter requires 1500 units, a communication satellite requires 1500˜3000 units, a shipborne radar requires 15,000 units, and the demand is even greater in the future space station and spacelab. If all the T/R modules in the above-mentioned military equipment in our country are replaced by using reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates, the demand will be extremely large and therefore its military and economic significance cannot be estimated. It is reasonable to conclude that using reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates to replace the traditional packaging materials such as Kovar, W/Cu and Mo/Cu is a significant reform in upgrade and replacement of communication equipment products in aviation and aerospace industry in our country and has significant meaning to our country.
However, this type of new electronic packaging materials has a bottleneck technical problem, which is the problem of joint welding for encapsulation of outer casing of electronic modules. On one hand, the case cover and the case body has to be welded together. Meanwhile, overheating damage to the case bottom on which the chip is mounted has to be avoided during welding (The working temperature limit of the chip is only 170° C.). In addition, it is required that the passing of stringent tests of the welded joint for air tightness, shock vibration, moisture resistance and corrosion resistance can be ensured. The quality requirements are very strict and this is one of the recognized technical problem of the welding industry in our country and at international level at the moment.
The status of foreign research: There is no existing report in relation to welding of reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates. The status of national research: Jitai Niu of Harbin Institute of Technology, funded by the National Nature Science Fund in 1989, because the first in the country to carry out the research on welding for aluminum matrix composite materials. Since the late 1990s, a number of college teachers and research students have made useful attempt on welding possibility of low volume fraction reinforced aluminum matrix composite from which the results show as follows: since there exists a great difference in physical and chemical properties between the enhanced phase and the aluminum matrix substrate, the use of fusion welding method to obtain high quality welded joint is extremely difficult and defects such as porosity, slag inclusion, loose connection and poor penetration will occur. At the same time, under the high temperature condition during the welding process, interface reaction between silicon carbide and liquid aluminum will occur and needle-shaped harmful compound of C3Al4 is produced. Connection is only possible by using Al—Si welding wire or dilution bath of matrix alloy welding wire, however, the result is that the major component in the weld seam becomes aluminum alloy instead of aluminum matrix composite materials and therefore the strength coefficient of the welding joint is very low. After a few years, Jitai Niu invented a fusion welding method with in situ synthesis and reinforcement of weld (Chinese Invention Patent Number: ZL200510010266.5) which can significantly increase the strength of the joint. However, this method is only applicable to low volume fraction (<20%) reinforced aluminum matrix composite material and is not applicable to high volume fraction (50%˜70%) reinforced aluminum matrix composite material. In recent years, the team of Jitai Niu have invented diffusion welding, vacuum brazing, furnace brazing and resistance welding for reinforced aluminum matrix composite with high volume fraction of silicon carbide particulates and obtained a number of national patents. The team of Jiuchun Yan of Harbin Institute of Technology invented vibration welding and ultrasonic capillary welding and contributed to the new research idea of welding method. That said, the welding quality of electronic packaging for high end products which has very strict technological requirements is still not satisfactory. The major insufficiency is tightness of brazing joint and resistance against environmental conditions (crossover temperature, vibration, space radiation). In particular, these method fails to use in mass production and fails to achieve the technical requirement of stable production with high passing rate for quality control requirements. The fundamental reason is that a large number of SiC ceramic reinforced phase which contain ionic bond and covalent bond exist on the surface of the composite materials such that wetting by brazing filler with metal bond is very difficult and hence the brazing process is very difficult to carry out, thereby further studies for effective brazing process is required.